The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using eddy-current sensors. Characterization of bulk material condition includes (1) measurement of changes in material state caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from shot peening, roll burnishing, thermal-spray coating, or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these also includes detection of electromagnetic property changes associated with single or multiple cracks. Spatially periodic field eddy-current sensors have been used to measure foil thickness, characterize coatings, and measure porosity, as well as to measure property profiles as a function of depth into a part, as disclosed in U.S. Pat. Nos. 5,015,951 and 5,453,689.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
In many inspection applications, large surface areas of a material need to be tested. This inspection can be accomplished with a single sensor and a two-dimensional scanner over the material surface. However, use of a single sensor has disadvantages in that the scanning can take an excessively long time and care must be taken when registering the measured values together to form a map or image of the properties. These shortcomings can be overcome by using an array of sensors or an array of elements within a single sensor, as described for example in U.S. Pat. No. 5,793,206, since the material can be scanned in a shorter period of time and the measured responses from each array element are spatially correlated. However, the use of arrays complicates the instrumentation used to determine the response of each array element. For example, in one conventional method, as described for example in U.S. Pat. No. 5,182,513, the response from each element of an array is processed sequentially by using a multiplexer for each element of the array. While this is generally faster than scanning a single sensor element, there is still a significant time delay as the electrical signal settles for each element and there is the potential for signal contamination from previously measured channels.
For nondestructive testing of conducting and/or magnetic materials over wide areas, eddy current sensor arrays may be used. These eddy current sensors excite a conducting winding, the primary, with an electrical current source of a prescribed frequency. This produces a time-varying magnetic field at the same frequency, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks. When scanning over wide areas, these arrays may include several individual sensors, but each sensor must be driven sequentially in order to prevent cross-talk or cross-contamination between the sensing elements.
Eddy current arrays have also been disclosed in U.S. Pat. No. 5,262,722, however the implemented versions of these arrays use differential sensing elements. The use of differential sensing element, that essentially compare the response of two neighboring sensing regions, limits the capability to determine absolute properties of interest. These sensor arrays and conventional eddy current sensors are also highly sensitive to sensor position, requiring expensive automated scanners to build images of material properties for complex surface inspections. Differential sensors may also produce false indications on relatively rough surfaces, such as surfaces with fretting damage.
Aspects of the inventions described herein involve novel sensors for the measurement of the near surface properties of conducting and/or magnetic materials. These sensors use novel geometries for the primary winding and sensing elements that promote accurate modeling of the response and provide enhanced capabilities for the creation of images of the properties of a test material.
In one embodiment, sensor array designs are disclosed that permit the creation of property images when scanned over a material surface. In one embodiment, the drive winding includes at least one central conducting segment and parallel return segments located on either side to impose a periodic magnetic field of at least two spatial wavelengths in a test material, a linear array of sensing elements to sense the response to the test material properties, and at least one sensing element uses a magnetoresistive (MR) or giant magnetoresistive (GMR) sensor. Secondary coils can also be placed around one or more of the MR or GMR sense elements, in one embodiment. In another, these coils are connected in a feedback configuration, and, in one embodiment, act to maintain the magnetic field at the MR or GMR sensor at a prescribed level.
In another embodiment of a sensor array design, the drive winding includes at least one central conducting segment and at least one parallel return segment on either side, a linear array of sensing elements between the central segments and a return segment, and separate connections to each sensing element. The distance between the central segments and the return segments can be selected to align with features of interest in a test material, such as bolt holes. One embodiment includes two central conductors and a return path for each conductor, with equal distances between the central conductors and each return path. In another embodiment, a second linear array of sensing elements is placed between another pair of linear drive winding segments, parallel to the first linear array. In one form, each element in the first array is aligned with an element in the second array. In another form, elements in the first array are offset from the elements in the second array in a direction parallel to the linear drive winding segments. Preferably, this offset distance is one-half of the length of a sense element, which ensures complete coverage of the element in a direction perpendicular to the drive winding segments. In an embodiment, the linear arrays are equally distant from the central conductors. Differential measurements may also be taken in the response between elements in the first array and elements in the second array. The central conductors can be placed in the same plane as the sensing elements to improve the coupling with the sense elements.
In an embodiment, the conductivity and proximity of the sensing elements to the surface are measured to detect cracks. In another, the proximity of each sensing element to the test material surface is used to determine surface roughness. In another embodiment, the sensing element response is used for health monitoring or condition assessment of a component. An embodiment also includes the use of a characteristic sensor response for a flaw and using that characteristic response to construct a filter. This filter can be applied to a response image to emphasize indications that are likely to be associated with flaws and suppresses indications unlikely to be associated with flaws.
In one embodiment, a single encoder determines the position of the array while scanning. In another embodiment, an automated scanner is used to move the array over a test material. In another embodiment, using modular fixtures with position encoders facilitates manual scanning of complex parts. In an embodiment, a template is used to align incremental scans over a test material so that images of the material properties over areas wider than the array width can be generated.
To facilitate the scanning of a sensor array over a material test surface, another embodiment includes the use of a fluid filled balloon. In an embodiment, this balloon is attached to a shuttle and the shuttle is shaped to approximately match the shape of the test material. In another embodiment, the sensor and balloon components are modularized and can be replaced rapidly. In one embodiment, the inspection is performed on the surface of a bolt hole. In another, the inspection is performed on the inside of an engine disk slot.
In another embodiment of a sensor array design, the drive winding includes at least one pair of parallel conducting segments to impose a magnetic field in the test material, a linear array of sensing elements, and separate connections to each sensing element. The distance between the parallel segments can be selected to align with features of interest in a test material. In one form, the linear array is placed between the parallel segments of the drive. Preferably, in another form, the array is placed outside the loop formed by the parallel segments of the drive. This also permits both the drive segments and the sense elements to be placed in the same plane.
In another embodiment, a second linear array of sensing elements is placed parallel to the first linear array of elements. This second array can be placed between the parallel segments of the drive winding, near a segment of the drive winding common with the first array. In another embodiment, the second array is placed outside the drive winding loop, opposite that of the first array. The distances between the linear arrays and the drive winding segments can be selected to be the same or different. In one embodiment, each element in the first array is aligned with an element in the second array. In another embodiment, elements in the first array are offset from the elements in the second array in a direction parallel to the linear drive winding segments. Preferably, this offset distance is one-half of the length of a sense element.
In an embodiment for the sensor array, the locations of the sensing elements in a direction parallel to the drive segments and the sensing element size can be made non-uniform to provide a higher image resolution over specific material test areas. In another embodiment, the sensor array can be fabricated onto a flexible substrate so that the sensor can conform to the shape of the test material. Alternatively, the sensor array can be fabricated onto a rigid substrate. With either substrate material, measurements of the material properties can be performed in a noncontact fashion. In an embodiment, at least one of the sensing elements includes a MR or GMR sensor. In one form, these sensing elements also include a secondary coil. In another form, the secondary coil is used in a feedback configuration.
In yet another embodiment, a sensor array design comprises two parallel linear rows of sensing elements on opposite sides of a central conductor for detecting cracks on each side of a feature. In one embodiment this feature is a fastener in an aircraft skin. In another embodiment, multiple frequency measurements are used to remove interference cause by the feature itself to isolate and emphasize the response of the crack. An embodiment also includes using the sensor response from a sensing element to create a characteristic response for a flaw and to construct a filter. This filter can be applied to a response image to emphasize indications that are likely to be associated with flaws and suppresses indications unlikely to be associated with flaws. In one form, the flaw is a crack. In another, the flaw is a buried inclusion.